The present invention relates to a method of determining print starting positions for an impact type dot printer having a printing head movable for printing in spacing increments in a direction perpendicular to the direction in which a recording medium such as printing paper is fed along.
Impact type dot printers are known which include a printing head having an electromagnetically drivable print hammer extending substantially in the direction in which a recording medium is fed along, and a platen rotatably disposed in confronting relation to the print hammer with the recording medium extending therebetween and having a plurality of axial ridges or projections on its circumference. The print hammer is selectively actuatable to hit the projections one at a time for selectively forming dots on the recording medium to print dot-matrix characters thereon.
Unlike wire printers and other types of printers, the positions of printed dots are determined by the relative positional relationship between the print hammer and the projections on the platen. Print timing pulses are generated by a dot sensor attached to a transmission gear train and energizable in synchronism with rotation of the platen. Such a print timing signal contains a pulse-free blank which is used as a reference to detect the rotational position of the projections on the platen with respect to the print hammer. More specifically, one of the projections on the platen is arranged to meet the print hammer on a first row of dot matrices when the first print timing pulse subsequent to the pulse-free blank arrives. The printing head is movable in spacing increments in synchronism with the platen at a certain relative speed through the transmission gear train, a clutch mechanism, and a carriage. A home sensor detects when the printing head leaves a home position. A print starting position is determined by using as a reference the pulse-free blank which arrives at first after the printing head has left the home position. Since there are some backlashes inherent in the transmission gear train and other intermediary mechanisms, the signal generated by the home sensor and the signal produced by the dot sensor undergo a relative phase difference, which has resulted in different print starting positions each time the printing head is moved in spacing increments, and hence in laterally shifted character positions staggered on lines. It has been customary practice to solve the foregoing problem by fabricating the parts of the dot printers with increased precision to remove unwanted backlashes as much as possible. Such an approach, however, is disadvantageous in that the dot printers become more costly, and it is almost impossible in reality to get rid of mechanical backlashes completely.